Meter and method of calibration

ABSTRACT

A D&#39;&#39;Arsonval type moving coil meter with linear scale deflection characteristics. The magnetic circuit of the meter is such that the sum total of flux which cuts the coil is the same for any operative position of the coil. Correspondingly, angular deflection of the coil is in direct proportion to the current in the coil. The magnetic circuit includes a magnet having fixed locating surfaces thereon which cooperate with fixed locating surfaces on a yoke. This locating arrangement assures that the sum total of flux which cuts the coil is the same for any operative position of the coil and further assures repeatable accuracy in successively assembled meters. The meter is calibrated by magnetizing the magnet to obtain full scale deflection when rated current is applied to the meter terminals. Where a calibrating resistor is used, the resistor is connected to the meter prior to calibration with the result that accuracy of the meter is assured without the need for using a precision resistor.

United States Patent 1 [111 3,757,217

Pearson 1 Sept. 4, 1973 METER AND METHOD OF CALIBRATION [75] lnventor:David B. Pearson, Raritan, NJ. ABSTRACT [73] Assignee: WestonInstrument, 1nc., Newark A DArsonval type moving coil meter with linearscale deflection characteristics. The magnetic circuit of the meter issuch that the sum total of flux which cuts the Med: 1971 coil is thesame for any operative position of the coil.

21 Appl. No.: 173,424

Primary Examiner Alfred E. Smith A ttorney-William R. Sherman, Jerry M.iresson et Correspondingly, angular deflection of the coil is in directproportion to the current in the coil. The magnetic circuit includes amagnet having fixed locating surfaces thereon which cooperate with fixedlocating surfaces on a yoke. This locating arrangement assures that thesum total of flux which cuts the coil is the same for any operativeposition of the coil and further assures re peatable accuracy insuccessively assembled meters. The meter is calibrated by magnetizingthe magnet to obtain full scale deflection when rated current is appliedto the meter terminals. Where a calibrating resistor is used, theresistor is connected to the meter prior to calibration with the resultthat accuracy of the meter is assured without the need for using aprecision resistor.

17 Claims, 7 Drawing Figures METER AND METHOD OF CALIBRATION Thisinvention relates generally to an electric meter of the moving coil typehaving linear deflection characteristics, and particularly to such ameter with a magnetic circuit of unique construction. The invention alsorelates to a method of manufacturing and calibrating such a meter.

In the past there have been numerous attempts to provide a meter of forexample, the DArsonval type in which the meter has truly linearcharacteristics i.e., the deflection of the pointer is directlyproportional to the current applied to the meter. Such attempts haveincluded magnetic circuit constructions in which the magnet for themeter has adjustable shunts or is of multi-layer construction to permitadjusting the magnetic characteristics in the gap in which the coil ispositioned. Alternatively, such prior art arrangements have includedadjustably mounted magnets which are mounted so the position of themagnet can be changed relative to the remainder of the magnetic circuitto achieve the desired characteristics. In addition, in someconstructions, the magnetic circuit itself is shiftable relative to themagnet. However, even with constructions of this type, an electric meterwith truly linear scale deflection characteristics was rarely if everobtained, except by accident. In any event, it is still necessary withprior art constructions, to have at hand perhaps four or five diflerentscale faces, the graduations on which are determined by calibrating aseries of successively produced meter movements. Then, each movementwhich is made is tested individually to determine its characteristicsand one of the predetermined scales is fitted to the movement to providea meter with reasonable accuracy but which is still not linear.

Applicant, in accordance with this invention, has devised a meter and amethod of making same in which the meter is capable of mass production,assembly of the meter is vastly simplified, no mechanical adjustment ofthe magnetic circuit is necessary, and the meter is quickly and rapidlycalibrated by the technique of properly magnetizing the meter magnetafter the meter is assembled.

The technique of magnetizing the meter magnet after the meter isassembled is performed while a current of predetermined value energizesthe meter coil to assure that for example, full scale deflection isobtained when the rated current for the meter movement is passed throughthe coil. By virtue of the linear scale deflection characteristics ofthe meter the meter can be calibrated at virtually any location on thescale, for example, onehalf scale deflection or full scale deflection.The technique however is the same in that the meter magnet is magnetizedafter the meter is assembled and the extent that the permanent magnet ismagnetized is precisely that required to provide the desired scaledeflection for the current in the coil.

Since the deflection characteristics of the meter are linear, the samemeter movement can readily be used with scales of at least severaldifferent sizes, and only one scale of each size is required for aparticular application of the meter. in contrast, in the past, four orfive scales of each size were required so the proper scale could beselected to provide an accurate meter. In order to adapt the meter toscales of different sizes it is merely necessary to provide a pointer ofthe proper length and a counterweight of the proper weight to balancethe pointer. In any event, since the angular spacing of the graduationson all the scales is the same, regardless of size, for a particularapplication of the meter, only one scale of each size is required incontrast to the four or five scales of each size required by the priorart meters.

An additional advantage of this magnetizing-afterassembly technique isthat meters of different sensitivities can be readily provided byconnecting an appropriate resistor to the meter coil before the magnetis magnetized. Hence, different meters of several differentsensitivities can be obtained from a basic meter movement merely byselecting an appropriate resistor and connecting same either in serieswith or in shunt across the meter coil. Extreme accuracy of theresulting meter is assured by connecting the resistor to the coil priorto magnetizing the magnet, and as a result, a meter of excellentaccuracy is assured even though the resistor is not a precisionresistor. By magnetizing the magnet after the resistor is connected tothe meter coil the tolerances of the resistor are meaningless since themeter is magnetized so the coil will deflect a predetermined distancefor a predetermined current in the coil.

The unique linear characteristics of the meter of this invention areobtained by so shaping and constructing the permanent magnet and theremaining portions of the magnetic current that the total flux whichcuts the coil is the same for all positions of the coil through whichthe coil rotates. By virtue of this construction, the electromagnetictorque acting on the coil to cause the coil to pivot is directlyproportional to the current applied to the coil. As a result, the metermovement exhibits true linear deflection characteristics, and even thetaut band or return spring for the meter, depending on the type ofsuspension used for the coil, need not be of precision manufacture, butneed only have linear deflection characteristics, since all variablesare compensated for by the final magnetizing of the permanent magnet.

The unique magnetic circuit of the meter of this invention includes apermanent magnet which is generally elliptical and which hasdiametrically opposed locating faces at opposite sides thereof toprecisely locate the magnet in a ring shaped yoke. Advantageously, theyoke is diametrically split and is constructed from two semi-circularhalf shells to provide the desired hollow cylindrical yoke. Hence, themagnetic circuit includes two generally crescent shaped air gaps throughwhich opposite sides of the moving coil extend. Since the flux densityin each of these air gaps is uniform, a predetermined current in thecoil will cause the coil to pivot a predetermined angular amount againstthe restoring action of the suspension system. Hence, it is onlyrequired that the return spring or taut band have linear deflectioncharacteristics, and since such characteristics are inherent in springssuch as torsion springs and spiral springs, linearity of the meter isassured.

Correspondingly, an object of this invention is to provide a meter ofthe moving coil type having cooperating fixed locating surfaces on ayoke and magnet of a configuration to provide linear deflectioncharacteristics such that the angular deflection of the coil is directlyproportional to the current in the coil.

Another object is a linear scale meter in which a permanent magnet and atwo piece soft iron yoke cooperate to define a flux gap in which the sumtotal of flux which cuts a coil in the gap is the same at any angularposition of the coil.

Another object is a meter with linear deflection characteristics inwhich a permanent magnet core of generally elliptical configuration isprecisely located within a ring shaped yoke by virtue of cooperatingfixed locating surfaces on the yoke and magnet, the locating surfacesmagnetically connecting the yoke and magnet such that the flux densityin the portions of the gap through which the coil deflects issubstantially uniform.

Another object is a meter of the DArsonval type in which a generallyrectangular coil extends through opposed gaps between a two piece yokeand a permanent magnet core, in which the portions of the coil in thegaps are exposed to a uniform flux density, and in which this uniformdensity is substantially the same at all angular positions of the coilfrom a zero position to a full scale position of the coil.

A further object is a unique method of assembling and calibrating ameter by magnetizing a permanent magnet core after the meter isassembled while applying a predetermined current to the meter coil, inthe environment of a meter with linear scale deflection characteristics,so the deflection of the coil of the calibrated meter is directlyproportional to the current in the coil.

A further object is a unique method of calibrating a meter using anon-precision resistor by magnetizing a permanent magnet core to theextent necessary to obtain a predetermined scale deflection as a resultof application of a predetermined current to the meter coil, whereby,non-precision characteristics of the resistor are automaticallycompensated for by the magnetizing operation.

Numerous other objects, features and advantages of this invention willbecome apparent with reference to the accompanying drawings which form apart of the specification and in which:

FIG. 1 is an exploded pictorial view showing the meter of thisinvention;

FIG. 2 is a partial plan view in partial section showing a taut bandsuspension arrangement for suspending the moving coil of the meter;

FIG. 3 is a view corresponding to FIG. 2, but showing a pivot bearingarrangement for mounting the moving coil for pivotal movement;

FIG. 4 is a partial view in vertical section of the meter movement takenalong line 44 of FIG. 1, and showing the magnetic circuit of the meter;

FIG. 5 is a view in section taken along line 5-5 of FIG. 4;

FIG. 6 is a diagrammatic view of apparatus for calibrating the meter;and

FIG. 7 is a view corresponding to FIG. 6 and showing an assembled meterpositioned in the apparatus of FIG. 6 for calibration.

Referring now to the drawings in detail and particularly to FIG. 1,there is shown a meter 1 constructed in accordance with this invention.As shown at FIG. 1, the meter includes a cover 2, a meter movement 3, acase 4, a scale 5 and a terminal adapter assembly 6. In addition, thereis a front zero adjust knob '7 which is adapted to be mounted forrotation in an opening 8 in cover 2. The various parts of the meter sofar described are adapted to be assembled by sliding and/or snappingthese parts together, with the exception of the terminal adapter 6 whichis held in position by the screw 9, and

which is only used where a particular terminal arrangement for the meteris needed. The various parts that make up meter movement 3 are assembledon a base 10 of insulating material.

The meter movement 3 includes a magnetic circuit 1 1, a moving coilassembly 12, a from support element 13 and a rear support element 14.The support elements 13 and 14 are mounted on base 10 for limitedrotation to provide for adjusting the zero position of pointer 15.Pointer 15 is mounted on coil assembly 12 by a support structure 16integral with a coil form part of the coil assembly.

Coil assembly 12 can be supported for pivotal movement by the taut bandsuspension arrangement shown at FIGS. 1 and 2, or alternatively can besupported by the pivot bearing arrangement shown at FIG. 3. Withreference to FIGS. 1 and 2, coil assembly 12 includes an upper coil formpart 18, and a lower coil form part 19 in an insulated relation to theupper coil part, with the parts held together by the insulated coil 20.End 21 of coil 20 is electrically connected to upper coil form part 18at a terminal 22. End 23 of coil 20 is electrically connected to lowercoil form part 19 of a terminal 24.

At the front of lower coil form part 19 is a taut band support finger25. A taut band 26 extends between finger 25 and a taut band supportfinger 27 integral with front support element 13. Similarly, there is asupport finger 28 at the rear of upper coil form part 18. A taut band 29extends between finger 28 and a support finger 30 integral with rearsupport element 14.

Front support element 13, rear support element 14, upper coil form part18 and lower coil form part 19 are all formed from a springy materialwith good electrically conducting characteristics, such as phosphorbronze. In addition, taut bands 26 and 29 are formed from anelectrically conducting material and take the form of a thin flat ribbonwhich electrically connects the respective coil form parts to the frontand rear support elements. By virtue of this arrangement, end 21 of coil20 is electrically connected to rear support element 14 and end 23 ofcoil 20 is electrically connected to front support element 13.

As shown at FIG. 1, conductors 31 and 32 are mounted respectively onopposite sides of base 10, which is integrally molded from a plasticwith good electrically insultaing characteristics. Conductor 31 iselectrically connected to front support element 13 by a flexible wire 33which permits limited rotation of the front support element so the zerodeflection point of the meter can be adjusted, for example, by rotatingfront zero adjust knob 7 which has gear teeth thereon and which teethmesh with teeth on the periphery of the front support element 13 whenthe meter is assembled. Where it is desired to provide a meter with asensitivity different from the sensitivity obtained with the coil 20, aresistor 35 can be connected in series between wire 33 and conductor 31.Alternatively, a shunt resistor 35' can be connected between conductors31 and 32. Where resistor 35 is connected in series, there is of courseno direct connection between wire 33 and conductor 31, but instead, wire33 is connected to one lead of resistor 35 and its other lead isconnected to conductor 31. The front of the base has an integral groove34 to mechanically retain resistor 35 against movement.

Rear support element 14 is connected to conductor 32 by a flexible wire34, so zero adjustment can be made by rotating the rear support element,via a suitable rear zero adjustment knob (not shown), if it is desiredto have access to the zero adjuster from the rear of the meter.

By virtue of this arrangement, current or voltage applied to theconductors 31 and 32 energizes coil of the meter. When the meter isassembled, the tail portions of the conductors 31 and 32 extend throughthe rear wall of body 36 of the casing so they are exposed for makingdesired electrical connections or for connection to adapter assembly 6.

Alternatively, a pivot bearing arrangement, such as shown at FlG. 3 canbe used to mount coil assembly 12 for pivotal movement. in thisarrangement, coil assembly 12 again includes the insulated upper andlower coil form parts 18 and E9 in insulated relation to each other anda coil 20 wound thereon. One end of coil 20 is connected to terminal 22and the other end 23 is connected to terminal 24. in the pivot bearingarrangement, front element 13 is really a front connector element andrear element 14' is also a connector element since these front and rearelements do not physically support coil assembly 12 for rotation, butonly provide for zero adjustment and electrical connection of the upperand. lower coil form parts to the meter terminals. The coil assembly 12is mounted for pivotal movement by a front pivot stud 37 and a rearpivot stud 38 which extend respectively through openings in the frontand 57 so the magnet is maintained in precise alignment with the centralaxis of the yoke ring.

The yoke ring 51 has smooth cylindrical surfaces 60 and 61 which facetoward the respective outside surfaces 62 and 63 of magnet 50. Whilesurfaces 62 and 63 of the magnet are arcuately curved and the body ofthe magnet has a uniform cross-sectional configuration, the surfaces 62and 63 are not precisely cylindrical, but are instead of ellipticalcurvature. Surface 60 cooperates with surface 62 to form a generallycrescent shaped gap 64 through which a side 65 of coil assembly 12extends. Similarly, surface 61 and 63 cooperate to define a generallycrescent shaped gap 65 through which side 66 of the coil assemblyextends. Of extreme significance is the fact that there is asubstantially uniform flux density in gap 65 throughout at least theangle of travel 67 of the coil assembly 12. Hence, the portion of coil20 on side 66 of the coil is cut by the same number of flux lines 68 atany position within gap 65 within the angle of travel 67. Similarly,there is a substantially uniform flux density in the gap 64 and theportion of coil 20 on side 65' is exposed to or cut by the I same numberof flux lines regardless of its position rear elements, but do notinterfere with rotation of these elements to zero adjust the meter. Theinner pointed tip 39 of stud 37 seats in a V-shaped depression in afront pivot bearing 40 supported on coil assembly 12. Similarly, theinner pointed tip 41 of stud 38 seats in a V-shaped depression in rearpivot bearing 42. These pivot bearings and studs mount coil assembly 12for pivotal movement. A spirally wound front coil return spring 63 hasits outer end electrically and mechanically secured to front supportelement 13' by a tab 44. The inner end of spring 43 is electrically andmechanically secured to lower support element 19 by a tab 45. Spring 43electrically connects lower coil form part 119 to front element 13.There is also a spirally wound leaf spring 46 at the rear of coilassembly 12. The outer end of the spring is electrically connected by atab 47 to rear element i4 and the inner end of the spring is connectedto upper coil form part 18 by a tab 43.

FIG. 4 shows a preferred embodiment of the magnetic circuit 11 of thisinvention. The magnetic circuit 11 includes a magnet core 50 mountedwithin a yoke ring 51 which includes a semi-cylindrical upper yokeelement 52 and a semi-cylindrical lower yoke element 53. The yokeelements are identical to each other, and the element 53 is merely theelement 52 turned 180 circumferentially. The yoke elements have matingabutment steps 54 and 55 which inter-engage, as shown at FIG. 4, to formthe diametrically split yoke ring 51. Rectangular slots 56 and 57, eachof uniform depth, are formed respectively in the upper and lower yokeelements to precisely locate the upper and lower rectangular ends 58 and59 of magnet 50 within the yoke ring 511. The yoke and magnet are sodimensioned, that magnet 50 is clamped between yoke elements 52 and 53when the yoke elements are seated in the base, as shown at FIGS. 1 and4. The width of ends 58 and 59 of the magnet is the same as the width ofslots 56 and within the angle of travel 67 of the coil assembly. It isto be appreciated that the flux density in gap 64 need not be the sameas the flux density in gap 65. In addition, the flux density in thesegaps can, of course, be different in different planes perpendicular tothe axis of yoke ring 51 (which corresponds to the axis of rotation ofcoil assembly 12) without adversely affecting the linearity of themeter. It is however quite significant that the sum total of flux whichcuts the sides 65' and 66 be the same throughout the angle of travel 67of coil assembly 12 in each gap. This is accomplished by making themagnet and yoke so the flux acting to rotate the coil assembly issubstantially the same for the side 66 at any position within its angleof travel and the flux acting on side 65' is substantially the same atany position within its angle of travel. A deviation of less than 2percent in the uniformity of the flux is permissible without adverselyaffecting the linear characteristics of the meter, although for veryaccurate meters this deviation is kept at less than one-half percent.

Magnetic circuit 11 is rigidly held in position on base 10 by legs 69and 70 integral with the base and which have downwardly facing latchedges 71 and 72 respectively, which engage horizontal faces of notches73 and 74 respectively in the upper yoke element 51 to hold the magneticcircuit firmly in position against a support pad 75 formed on the bottomof base 10. The magnetic circuit is located in a fore and aft directionon base 10 by a rear locating post 76 which engages the rear faces ofboth the magnet and lower yoke element 53, and a front locating post 77which engages a locating surface 78 in a notch 79 at the front of themagnet and which also engages the front surface of the lower yokeelement at a location on each side of a notch 79 in the lower yokeelement. The sides of locating post 77 position lower yoke element 53circumferentially on the base 16. There is only one notch 79 in themagnet. In addition to providing a locating surface 78, the notch 79provides for mounting the magnet in the base 10 in only one position,namely the position shown, for example, at FIG. 5 where the notch isadjacent post 77. The top edge 81 of the projection 82 of post 77prevents installing the magnet in any other position, such as upsidedown or with the notch 79 facing toward post 76. Hence, the magnet isalways installed in the same position in successively constructed metersand correspondingly, repeatable accuracy of the meter is assured.

The embodiment of P108. 4 and 5 provide for assembling the meter byfirst pressing lower yoke element 53 into the base, then installing coilassembly 12, then inserting magnet 50, and finally pressing upper yokeelement 52 into position on the lower yoke element so the arms 69 and 70snap over the surfaces 73 and 74 to hold the magnet and yoke element inposition.

Magnet 50 is formed from a material with good magnetically retentivecharacteristics, such as LODEX, whereas the yoke ring 50 is formed frommagnetic material with non-retentive characteristics and low magneticreluctance, such as soft iron.

After the meter is balanced and is completely assembled in case 4, knob7 is manipulated to set pointer to its zero position on scale 5, thenthe meter is calibrated. Such calibration is accomplished with the calibrating equipment 89 of FIG. 6. The equipment includes an electro-magnet90 with pole faces 91 and 92 spaced apart a distance slightly greaterthan the distance between the sides of body 36 of case 4. Electromagnet90 is energized by energizing coil 93 from a direct current source 94 byclosing switch 95. There is a second electro-magnet 96 with opposed polefaces 97 and 98 which extend transverse to the pole faces 91 and 92 so agenerally rectangular opening is formed between the pole faces 91, 92and 97, 98. Electro-magnet 96 has a coil 99 which is energized from asource of alternating current 100 by closing a switch 101. The distancebetween pole tips 97 and 98 is only slightly greater than the height ofbody 36, so the body can be positioned between the pole tips after themeter is assembled. The calibration equipment also includes a powersource 102 with leads 103 and 104 for connection to the terminals of themeter to energize coil of the meter. Power source 102 is preferably adirect current source, but can also be an alternating current source ifthe assembled meter includes rectifiers for the measurement ofalternating current.

FIG. 7 shows the assembled meter in position in the calibrationequipment. It will be observed that the scale 5 has graduations 107thereon which are spaced apart precisely the same angular distance. Inan accurate meter, the pointer 15 must swing from the zero position 108to the full scale position 109 when a predetermined current is appliedto the terminals of the meter. In addition, with the linear meter ofthis invention, pointer 15 moves to a half scale position 110 when onlyhalf the current is applied to the terminals. The sensitivity of themeter determines the current required to move the pointer to the fullscale position 109. For example, if the sensitivity of the meter is 100micro-amps, applying 100 micro-amps to the terminals of the meter mustcause the pointer to swing to the full scale position 109.correspondingly, applying 50 micro-amps to the meter terminals mustcause the pointer to swing to the half scale position 110 and at anyintermediate position between zero position 100 and full scale position109 the deflection of the pointer must be directly proportional to thecurrent applied to the terminals.

For purposes of economy of construction, it is in many instancesdesirable to use the same coil assembly 12 with coil 20 of the same wireand same number of turns for meters of several different sensitivities.For

example, the meter can be so designed that the coil 20 deflects to thefull scale position 109 when 100 microamps of current are in the coil.Hence, where it is desired to provide different sensitivities, such as200 micro-amps or 500 micro-amps, resistors such as the resistors 35 or35 can be connected either in series with or in shunt across the coil tochange the sensitivity of the meter. In the past, such resistors havebeen of precision manufacture to assure that the proper portion of thecurrent applied to the meter terminals flows through the coil 20 todeflect the coil and pointer. In accordance with this invention, theseresistors need not be of precision manufacture since the meter iscalibrated after such resistors, when used,are connected to the coil.

A feature of both the taut band suspension of FIGS. 1 and 2 and thespiral spring-pivot bearing arrangement (FIG. 3), which is quitesignificant to the meter of this invention, is that the spring and tautband each resist deflection of the coil assembly in direct proportion tothe extent of angular deflection of the assembly from a zero position.Such resistance to deflection in direct proportion to the extent of thedeflection is an inherent characteristic of spiral and torsion typesprings. Hence, both the taut band suspension and the spiral springarrangement exert a restoring force on the coil which is directlyproportional to the extent of deflection of the pointer 15 from its zeroposition 108. The deflection of coil 20, resulting from applying acurrent to the coil, is directly proportional to the current in the coilbecause the flux density in each of the gaps 64 and 65 is the same forany position of the coil within its angle of travel 67. Since both thespring force resisting deflection and the force resulting fromenergizing the coil are linear, the deflection of the coil in responseto a current in the coil is also linear.

Calibration of the assembled meter will now be explained with referenceto FIG. 7. First, the meter is positioned in the calibrating equipmentwith the body 36 in the opening between the respective pole faces 91,92, and 97, 98. Then, leads 103 and 104 are connected respectively tothe terminal conductors 31 and 32 of the meter. Then, switch 95 isclosed to energize electromagnet to magnetize magnet 50 transverse toits axis and in a generally horizontal direction. Advantageously, themagnet 50 is over-magnetized by this magnetizing step so applying therated current to the meter terminals from the power source 102 causespointer 15 to deflect to a position 1 11 which is beyond its full scaleposition. Then, magnet 50 is de-magnetized in a stepby-step fashion byenergizing electro-magnet 96 from its power source 100, until pointer 15reaches the full scale position 109. Since the calibration is performedafter the meter is completely assembled, variables, such as thetolerances of resistors 35 and 35' (when used) or the effects ofrectifiers (when used) are compensated for by this calibrationtechnique. In addition, since the deflection characteristics of themeter are linear, applying half the rated current will cause the pointerto deflect to the half scale position 110. By virtue of this technique,and the construction described herein, a meter with linear deflectioncharacteristics is provided which has good accuracy (on the order of 2percent) and which is capable of manufacture by mass production methods.A distinct advantage of the linear deflection characteristics is thatthe number of different scale plates required by prior art meters isdispensed with, since all the scale plates have the same linear orequally spaced graduations whenever a linear characteristic is measuredwith the meter.

While a preferred embodiment of the meter has been described in detail,and while a preferred method of calibration has also been described, itis to be understood that numerous changes and variations can be madewithout departing from the scope of this invention as defined herein andin the appended claims.

What is claimed is:

l. A moving coil measuring instrument wherein deflection of the coil isdirectly proportional to the current in the coil, and wherein theinstrument is adapted to be calibrated after assembly by magnetizing amagnet of the instrument to a required extent; comprising incombination, an adjustment free magnetic circuit comprising a pair ofunitary yoke pieces abutting each other to form a ring shaped two pieceyoke, and a core within and magnetically connected to said yoke, one ofsaid yoke and core being of magnetic material and the other of said yokeand core including a fixed unitary permanent magnet; said yoke and corehaving fixed, abutting, cooperating locating surfaces for mounting saidyoke and core in only one predetermined aligned position with respect toeach other; opposed surface means of said yoke and core defining a coilreceiving gap therebetween; a. coil assembly; means mounting said coilassembly with a portion thereof extending through said gap and formovement through a predetermined angle of rotation at least as great asthe angular deflection of the coil from a zero position to a full scaleposition; said opposed surface means cooperating with said magnet toprovide flux which cuts the coil, the sum total of said flux which cutsthe coil being substantially the same for any position of said coilwithin said predetermined angle of rotation; whereby deflection of saidcoil in said gap is directly proportional to the current in said coil.

2. A measuring instrument according to claim 1 wherein said opposedsurface means of said yoke and core cooperate to provide a crescentshaped gap; and the flux density in said gap is substantially uniformthroughout said predetermined angle of rotation of said coil.

3. A measuring instrument according to claim 1 wherein said cooperatinglocating surfaces include first and second fixed locating surfaces onsaid core, said yoke includes first and second fixed locating surfacesin engagement respectively with said locating surfaces of said core;whereby, said core is in a predetermined fixed position relative to saidyoke.

4. A measuring instrument according to claim 3 wherein said first andsecond locating surfaces of said core are generally rectangular; andsaid first and second locating surfaces of said yoke include rectangularslots to receive said rectangular portions of said core whereby, saidcore is maintained in a fixed position within said yoke.

5. A measuring instrument according to claim 1 wherein said yoke is ofuniform cross-sectional configuration; said core is of uniformcross-sectional configuraton; said opposed surface means defining saidgap include a smooth arcuately curved inner surface of said yoke and asmooth arcuately curved exterior surface of said core.

6. A moving coil measuring instrument wherein deflection of the coil isdirectly proportional to the current in the coil, comprising incombination, a magnetic circuit comprising an adjustment free hollowyoke of magnetic material, and a unitary permanent magnet in fixedabutting relation with respect to, and clamped within said yoke; saidyoke having an inner surface in opposed relation to an outer surface ofsaid magnet, said inner surface and said. outer surface defining a coilreceiving gap therebetween; a coil assembly; means mounting said coilassembly with a portion thereof extending through said gap and formovement through a predetermined angle of rotation at least as great asthe angular deflection of the coil from a zero position to a full scaleposition; said mountng means including means exerting an opposing forceon said coil proportional to the angular deflection of said coil fromsaid zero position; said opposed surfaces of said magnet and yokeproviding a flux gap in which the sum total of the flux which cuts thecoil is substantially the same for any position of said coil within saidpredetermined angle of rotation of the coil assembly so that themagnetic circuit requires no adjustment but can be calibrated bypermanently magnetizing the permanent magnet after assembly; whereby,deflection of said coil assembly in said gap is directly proportional tothe current in the coil.

7. A measuring instrument according to claim 6 wherein said meansmounting said coil assembly for rotation includes taut band suspensionmeans extending between said coil assembly and a supporting base.

8. A measuring instrument according to claim 6 wherein said meansmounting said coil assembly for rotation includes pivot bearing meansextending between said coil assembly and a supporting base.

9. An instrument according to claim 6 wherein said magnet includes meansto permit installation thereof in only one predetermined orientedposition.

10. An instrument according to claim 9 wherein said instrument includesa base having a positioning surface, and said means on said magnetincludes a notch cooperating with said surface to permit mounting saidmagnet in only one predetermined oriented position.

11. A measuring instrument according to claim 6 wherein said yoke iscomprised of two semi-circular yoke elements, and said magnet is clampedbetween said yoke elements.

12. A method of making a linear scale meter of predetermined sensitivitycomprising in combination, the steps of providing a movable coil;providing an adjustment free magnetic circuit having a permanent magnetand a flux gap and with a portion of the coil in said gap, said magneticcircuit including an adjustment free two piece yoke, said gap and coilcooperating so the sum total of the flux in the gap which cuts the coilis the same for any operative position of the coil in the gap; changingthe magnetization of said permanent magnet with a magnetizing sourceseparate from the meter while applying a predetermined current to saidcoil until said coil deflects to a predetermined calibration position;and removing the meter from the magnetizing source.

13. A method according to claim 12 wherein said step of providing a coilincludes connecting a resistor between said coil and a terminal of themeter before changing the magnetization of said magnet; and said step ofapplying a predetermined current to said coil includes applying saidcurrent to the meter terminals.

14. A method according to claim 12 wherein said steps of providing amagnetic circuit and providing a coil include providing an assembledmeter movement, and wherein the method further includes placing saidmovement in a casing; mounting a linear scale adjacent said pointer; andadjusting the zero position of said pointer prior to changing themagnetization of said magnet.

15. A method of calibrating an electric meter of predeterminedsensitivity comprising in combination, the steps of, providing a movablecoil; providing a magnetic circuit having a permanent magnet and a fluxgap, and with a portion of the coil in said gap; connecting a resistorbetween the coil and a terminal of the meter; subsequently changing themagnetization of said permanent magnet with a magnetizing sourceseparate from the meter while applying a predetermined current to saidcoil through said resistor until said coil deflects to a predeterminedcalibrated position; and removing the meter from the magnetizing source.

16. A method according to claim 15 wherein said gap and coil cooperatewith each other so the sum total of flux in the gap which cuts the coilis the same for any operative position of the coil in said gap.

17. A method according to claim 15 wherein said step of connecting saidresistor between said coil and a terminal of the meter includesmechanically supporting said resistor on a supporting base of saidmagnetic circuit.

1. A moving coil measuring instrument wherein deflection of the coil isdirectly proportional to the current in the coil, and wherein theinstrument is adapted to be calibrated after assembly by magnetizing amagnet of the instrument to a required extent; comprising incombination, an adjustment free magnetic circuit comprising a pair ofunitary yoke pieces abutting each other to form a ring shaped two pieceyoke, and a core within and magnetically connected to said yoke, one ofsaid yoke and core being of magnetic material and the other of said yokeand core including a fixed unitary permanent magnet; said yoke and corehaving fixed, abutting, cooperating locating surfaces for mounting saidyoke and core in only one predetermined aligned position with respect toeach other; opposed surface means of said yoke and core defining a coilreceiving gap therebetween; a coil assembly; means mounting said coilassembly with a portion thereof extending through said gap and formovement through a predetermined angle of rotation at least as great asthe angular deflection of the coil from a zero position to a full scaleposition; said opposed surface means cooperating with said magnet toprovide flux which cuts the coil, the sum total of said flux which cutsthe coil being substantially the same for any position of said coilwithin said predetermined angle of rotation; whereby deflection of saidcoil in said gap is directly proportional to the current in said coil.2. A measuring instrument according to claim 1 wherein said opposedsurface means of said yoke and core cooperate to provide a crescentshaped gap; and the flux density in said gap is substantially uniformthroughout said predetermined angle of rotation of said coil.
 3. Ameasuring instrument according to claim 1 wherein said cooperatinglocating surfaces include first and second fixed loCating surfaces onsaid core, said yoke includes first and second fixed locating surfacesin engagement respectively with said locating surfaces of said core;whereby, said core is in a predetermined fixed position relative to saidyoke.
 4. A measuring instrument according to claim 3 wherein said firstand second locating surfaces of said core are generally rectangular; andsaid first and second locating surfaces of said yoke include rectangularslots to receive said rectangular portions of said core whereby, saidcore is maintained in a fixed position within said yoke.
 5. A measuringinstrument according to claim 1 wherein said yoke is of uniformcross-sectional configuration; said core is of uniform cross-sectionalconfiguraton; said opposed surface means defining said gap include asmooth arcuately curved inner surface of said yoke and a smootharcuately curved exterior surface of said core.
 6. A moving coilmeasuring instrument wherein deflection of the coil is directlyproportional to the current in the coil, comprising in combination, amagnetic circuit comprising an adjustment free hollow yoke of magneticmaterial, and a unitary permanent magnet in fixed abutting relation withrespect to, and clamped within said yoke; said yoke having an innersurface in opposed relation to an outer surface of said magnet, saidinner surface and said outer surface defining a coil receiving gaptherebetween; a coil assembly; means mounting said coil assembly with aportion thereof extending through said gap and for movement through apredetermined angle of rotation at least as great as the angulardeflection of the coil from a zero position to a full scale position;said mountng means including means exerting an opposing force on saidcoil proportional to the angular deflection of said coil from said zeroposition; said opposed surfaces of said magnet and yoke providing a fluxgap in which the sum total of the flux which cuts the coil issubstantially the same for any position of said coil within saidpredetermined angle of rotation of the coil assembly so that themagnetic circuit requires no adjustment but can be calibrated bypermanently magnetizing the permanent magnet after assembly; whereby,deflection of said coil assembly in said gap is directly proportional tothe current in the coil.
 7. A measuring instrument according to claim 6wherein said means mounting said coil assembly for rotation includestaut band suspension means extending between said coil assembly and asupporting base.
 8. A measuring instrument according to claim 6 whereinsaid means mounting said coil assembly for rotation includes pivotbearing means extending between said coil assembly and a supportingbase.
 9. An instrument according to claim 6 wherein said magnet includesmeans to permit installation thereof in only one predetermined orientedposition.
 10. An instrument according to claim 9 wherein said instrumentincludes a base having a positioning surface, and said means on saidmagnet includes a notch cooperating with said surface to permit mountingsaid magnet in only one predetermined oriented position.
 11. A measuringinstrument according to claim 6 wherein said yoke is comprised of twosemi-circular yoke elements, and said magnet is clamped between saidyoke elements.
 12. A method of making a linear scale meter ofpredetermined sensitivity comprising in combination, the steps ofproviding a movable coil; providing an adjustment free magnetic circuithaving a permanent magnet and a flux gap and with a portion of the coilin said gap, said magnetic circuit including an adjustment free twopiece yoke, said gap and coil cooperating so the sum total of the fluxin the gap which cuts the coil is the same for any operative position ofthe coil in the gap; changing the magnetization of said permanent magnetwith a magnetizing source separate from the meter while applying apredetermined current to said coil until said coil deflects to apredetermined calibration position; aNd removing the meter from themagnetizing source.
 13. A method according to claim 12 wherein said stepof providing a coil includes connecting a resistor between said coil anda terminal of the meter before changing the magnetization of saidmagnet; and said step of applying a predetermined current to said coilincludes applying said current to the meter terminals.
 14. A methodaccording to claim 12 wherein said steps of providing a magnetic circuitand providing a coil include providing an assembled meter movement, andwherein the method further includes placing said movement in a casing;mounting a linear scale adjacent said pointer; and adjusting the zeroposition of said pointer prior to changing the magnetization of saidmagnet.
 15. A method of calibrating an electric meter of predeterminedsensitivity comprising in combination, the steps of, providing a movablecoil; providing a magnetic circuit having a permanent magnet and a fluxgap, and with a portion of the coil in said gap; connecting a resistorbetween the coil and a terminal of the meter; subsequently changing themagnetization of said permanent magnet with a magnetizing sourceseparate from the meter while applying a predetermined current to saidcoil through said resistor until said coil deflects to a predeterminedcalibrated position; and removing the meter from the magnetizing source.16. A method according to claim 15 wherein said gap and coil cooperatewith each other so the sum total of flux in the gap which cuts the coilis the same for any operative position of the coil in said gap.
 17. Amethod according to claim 15 wherein said step of connecting saidresistor between said coil and a terminal of the meter includesmechanically supporting said resistor on a supporting base of saidmagnetic circuit.